The present invention relates generally to portable phones and other electronic devices employing an external battery pack to provide power, and is particularly concerned with a protection circuit for preventing accidental short circuits of the exposed contacts of an external battery pack when disconnected from the phone or charging unit.
The back of a portable phone typically has spring-loaded contacts for mating engagement with flush contacts of an external battery pack when releasably secured to the phone. Various types of releasable fastening devices are used to secure such battery packs to portable phones. The battery pack fits into a charging unit with similar spring-loaded contacts when re-charging is required.
One problem with such external battery packs is that they may be accidentally short circuited by the user when not connected to the phone or charging unit. Most such battery packs, such as Li-ion battery packs, for example, have built in self protection circuitry for preventing overcharge or over discharge of the battery pack. However, when spare battery packs are carried loose in a user's bag, pocket, or purse, an accidental short circuit may occur where loose coins or keys connect the positive and negative exposed contacts. Such a situation may not activate the built in self protection circuit, since the thresholds for such short circuits must be higher than the operating discharge current under normal conditions, i.e. if the device draws peak currents at 1 amp, then the self-protection circuit of the battery pack must trip at currents above 1 amp. This will not activate the current self protection circuit provided in such battery packs.